The present invention relates to the detection of blocked or defective cooling holes in a gas turbine engine blade and, more particularly, to an apparatus and method for inspecting the cooling holes of a gas turbine engine blade by using infrared thermography.
Referring to FIG. 1, a gas turbine engine blade 10 has a multiplicity of cooling channels or holes 12 formed therein to permit cooling of the blade during engine operation. The cooling holes 12 extend from an exterior surface 14 of blade 10 into a hollow interior 16 or plenum of the blade 10. The blade hollow interior includes a plurality of interior walls 18 or baffles to direct cooling air, indicated by arrows 20, through the interior of blade 10 and out cooling holes 12 to create cooling air streams, indicated by arrows 22. Cooling air 20 absorbs heat within the interior 16 of blade 10 and also from the walls surrounding cooling holes 12 and cooling air streams 22 exiting holes 12 flow over the exterior surface 14 to further cool the blade.
In order to function properly, the cooling holes 12 must be constructed to a known configuration because the distribution of airflow must be controlled to achieve proper cooling of the blade during engine operation. Thus, the cooling holes must not be blocked or even partially blocked to provide sufficient and uniform cooling air distribution through blade interior 16 and across the exterior 14 of blade 10. Inspection of cooling holes 12 to detect blockages is difficult because of the small size of the holes; a typical hole diameter is about 12 mils (0.3 mm.). A wire or pin diameter gage may be used to inspect the cooling holes for blockage but this method is time consuming, tedious and labor intensive. Additionally, the wire or pin gauges can break and pieces of the gauge can become trapped within the interior plenum or the cooling hole and therefore block the hole.
A method and apparatus for inspecting cooling holes using infrared thermography is disclosed and claimed in U.S. Pat. No. 4,644,162, issued Feb. 17, 1987, assigned to the same assignee as the present invention, and incorporated herein in its entirety by reference. This patent discloses forcing a heated gas through a relatively cooler cooling hole, measuring the infrared signature of the cooling holes during the transient with a scanning infrared radiometer, and comparing the measured radiation intensity with a reference. The patent also contemplates forcing a cool gas through a relatively warmer channel and measuring the radiation intensity emitted by the cooling holes but the patent does not disclose the advantages derived by measuring the radiation intensity of the cooling holes during a heat-up cycle, when a heated gas is forced through the cooling holes and also during a cool-down cycle, when a chilled gas is forced through the cooling holes immediately after the heat-up cycle.
Observing only a heated gas forced through the cooling holes or only a cool gas forced through relatively warmer cooling holes can give rise to erroneous characterization of a blocked hole or channel. Additionally, inferences as to the specific nature of the cooling hole defect may be difficult to make by measuring the infrared signature during the transient of only either a heat-up cycle or a cool-down cycle independent of one another.